How to edit UVs for a mirrored effect on textures
For this, I dissolved the diagonal line going through the middle of my face. I then created a vertical loop cut down the centre of it, like so.

I then Went in the UV editor, selected all the edges of the left side of the face, and moved to be on the skull side of the texture.
All that's needed now is to download the UV: Magic UV Plugin, available in edit/preferences/add-ons, right click UV Unwrap Faces/Flip Rotate UV.
Done